The present invention relates generally to a system for regulating the high pressure component of a transcritical vapor compression system by employing an expander coupled to a fluid pumping device, such as a fan or a pump.
Chlorine containing refrigerants have been phased out in most of the world due to their ozone destroying potential. Hydrofluoro carbons (HFCs) have been used as replacement refrigerants, but these refrigerants still have high global warming potential. xe2x80x9cNaturalxe2x80x9d refrigerants, such as carbon dioxide and propane, have been proposed as replacement fluids. Unfortunately, there are problems with the use of many of these fluids as well. Carbon dioxide has a low critical point, which causes most air conditioning systems utilizing carbon dioxide to run partially above the critical point, or to run transcritical, under most conditions. The pressure of any subcritical fluid is a function of temperature under saturated conditions (when both liquid and vapor are present). However, when the temperature of the fluid is higher than the critical temperature (supercritical), the pressure becomes a function of the density of the fluid.
When a vapor compression system is run transcritical, it is advantageous to regulate the high pressure component of the system. By regulating the high pressure of the system, the capacity and/or efficiency of the system can be controlled and optimized.
In the prior art, the high pressure component of a vapor compression system has been regulated by adjusting an expansion valve located at the exit of the gas cooler, allowing for control of system capacity and efficiency. Suction line heat exchangers and storage tanks have also been employed to increase system capacity and efficiency.
A transcritical vapor compression system includes a compressor, a gas cooler, an expansion device, and an evaporator. Refrigerant is circulated though the closed circuit system. Preferably, carbon dioxide is used as the refrigerant. As carbon dioxide has a low critical point, systems utilizing carbon dioxide as a refrigerant usually require the vapor compression system to run transcritical. When the system is run transcritical, it is advantageous to regulate the high pressure component of the system to control and optimize the capacity and/or efficiency of the system.
An expansion machine is a work recovery device which extracts energy from the expansion process. The amount of energy available for extraction by the expansion machine is generally proportional to the refrigerant pressure drop between the gas cooler and the evaporator.
The expansion device is coupled to a fluid pumping device that pumps the heat exchange fluid (typically air or water) through the gas cooler. The heat exchange fluid is used to cool the refrigerant in the gas cooler. The fluid pumping device pumps fluid through the gas cooler at a rate which is related to the amount of energy extracted from the expansion process.
The system provides a self-controlling mechanism to regulate the refrigerant pressure in the gas cooler. When the refrigerant pressure in the gas cooler increases, the refrigerant pressure drop between the gas cooler and the evaporator increases, and the expansion machine extracts more energy from the expansion process. As the energy increases, the flowrate of the fluid pumping device increases, increasing the effectiveness of the gas cooler and decreasing the pressure of the refrigerant in the gas cooler. When the refrigerant pressure in the gas cooler decreases, the refrigerant pressure drop between the gas cooler and the evaporator decreases, and the expansion machine extracts less energy from the expansion process. As the energy decreases, the flowrate of the fluid pumping device decreases, decreasing the effectiveness of the gas cooler and increasing the pressure of the refrigerant in the gas cooler